Okay so we know Madara was born Indras next incarnate and his eyes had indras chakra in them.
After fighting Hashirama madara has to use Izanagi in his right eye pretty much deading that eye.
Madara then ate hashirama flesh and wherever he found another right eye Sharingan his eyes awakened into Rinnegan.
His eye should not have ascended. That was a foreign eye that did not possess Indras power.
Madara having double Rinnegan is a plot hole
After fighting Hashirama madara has to use Izanagi in his right eye pretty much deading that eye.
Madara then ate hashirama flesh and wherever he found another right eye Sharingan his eyes awakened into Rinnegan.
His eye should not have ascended. That was a foreign eye that did not possess Indras power.
Madara having double Rinnegan is a plot hole